1. Field of the Invention
A light emitting diode (LED) light source assembly for a billboard type sign, and more particularly an improved light spreading system therefor.
2. Related Art
A billboard is a type of signage in the form of a large outdoor advertising structure typically found along busy roadways and other high traffic areas. Billboards often show slogans, visuals and other indicia that are painted or otherwise applied to a large, usually rectangular, vertical surface. Billboard advertisements are designed to visually catch the attention of people passing by and quickly create a memorable impression. To be effective, a billboard sign must present indicia that are visually discernible from a great distance by viewers who may only have line-of-sight exposure to the sign for a very short period of time. During daylight hours, visibility is usually not an issue. However, during times of low-level ambient light, e.g., at night, it is usually necessary to shine a light onto the viewing surface of the billboard to in order to make the indicia visible to distant, transitory viewers.
In most instances, lighting of the billboard is accomplished by a horizontal row of light source assemblies mounted a few feet out from the large rectangular viewing surface either along its bottom or top edge. Traditionally, the lighting source has been an incandescent bulb set in an opaque housing with a clear cover. To disperse the light evenly across the viewing surface, reflectors may be placed behind the bulb inside the housing, and the cover may include light dispersing features like a Fresnel lens. Such traditional approaches were reasonably effective, but were relatively expensive to manufacture, expensive to maintain/repair, expensive to operate, and suffered relatively short life cycles between bulb replacements.
In recent years, incandescent lights have been increasingly replaced by LED (light emitting diode) devices due to the greater efficiency and longevity of an LED light source. High brightness, high efficiency LEDs have been developed to produce white light with an output sufficient for the development of practical lighting. Notwithstanding, LED applications have not successfully extended into billboard signage illumination due, at least in part, to difficulty obtaining an even light dispersion across the viewing surface. In particular, one type of LED light source a planar LED array consolidated onto a single light emitting chip—is well-known for its exceptionally high light output characteristics. However, planar LED arrays have not heretofore been successfully applied to billboard signage illumination applications. Fundamental differences between the emission of light from a planar LED array as compared with an incandescent light source have rendered the traditional light dispersion techniques—chiefly reflectors placed behind the bulb and/or a lensed cover—to be found generally ineffective for planar LED array light source technologies. For example, light output from planar LED array systems are substantially more concentrated along a unidirectional light emission beam, and hence more intense, than the omnidirectional light output from the ubiquitous filament(s) found in an incandescent source. The intense, unidirectional light emission beam from an LED light source, and in particular from a planar LED array configuration, results in a viewing angle so narrow that for billboard applications the traditional cover optics, Fresnel lensing, are not sufficiently effective to achieve the necessary light spread. Furthermore, cover optics, e.g., those crafted with Fresnel lensing, are expensive when compared with a non-lensed cover. Replacement of lensed covers due to damage and extended exposure to the elements contributes significantly to the high cost of traditional lighting solutions. Another obstacle to use of planar LED array technology in billboard signage applications has been the issue of heat management. It is well-known that LED light sources will fail prematurely without effective strategies for heat dissipation.
Thus, despite the recognized advantages of adapting outdoor signage lighting to LED's, there remains serious obstacles to successfully implementing LED concepts in this field due to fundamental differences in the respective light emission characteristics. There is therefore a need in the art for improvements in light dispersion technology sufficient to enable the use of planar LED array light source technology in such a way that results in cheap, robust, high-efficiency, and heat manageable outdoor signage illumination.